ACTIVE ELECTROMAGNETIC SHIELDING

Abstract: A cancellation circuit provides active electromagnetic shielding for canceling inductive noise in electrical circuits caused by electromagnetic flux. The cancellation circuit includes amplitude-adjustment and phase-adjustment circuits for adjusting amplitude and phase of electrical signals, and a means for combining electrical signals such that the effects of electromagnetic induction cancel. An electromagnetic pickup is provided with a cancellation circuit for canceling its response to electromagnetic flux. An electromagnetic drive coil is provided with a cancellation circuit for canceling electromagnetic flux in a predetermined region of space, and a compensation circuit is disclosed for compensating for frequency-dependent phase and amplitude variations in electrical pickup signals and transmitted electromagnetic flux. The cancellation and compensation circuits may be combined to provide a device that can simultaneously transmit and receive electromagnetic radiation.

Description: ACTIVE ELECTROMAGNETIC SHIELDING BACKGROUND OF THE INVENTION This application is related to application Ser. Nr. 08-097272. Filed July 23, 1993. The present invention relates to electromagnetic shielding for shielding electromagnetic pickups, other types of electronic equipment, and specific regions of space from electromagnetic radiation, and more particularly to active electromagnetic shielding for providing an electrical cancellation signal for canceling electromagnetic radiation or canceling the response of an electronic device to electromagnetic radiation. It has long been known that voltages are induced in all conductors exposed to changing magnetic fields regardless of the configuration of such conductors. Electromagnetic radiation will induce electrical signals in electronic devices according to the laws of magnetic induction. Thus it has been desirable in some applications of electronic instrumentation to reduce the inductive noise caused by electromagnetic radiation. A common method for providing electromagnetic shielding involves the use of passive electromagnetic shielding. A shield consisting of layers of high and low permeability material may be used to attenuate electromagnetic radiation passing through it However, this passive electromagnetic shielding adds substantial bulk and weight to the system which it shields. Another method for providing electromagnetic shielding is to utilize cancellation coils for generating a cancellation electromagnetic radiation in opposition to incident radiation produced by external sources in order to cancel the effects of the incident radiation. In U.S. Pat No. 5,066,891, Harrold presents a magnetic field sensing and canceling circuit for use with a cathode ray tube (CRT). Magnetic flux gate sensors provide output signals that are functions of detected fields. These signals are then used to control the current in the cancellation coils which produce a cancellation magnetic field. Harold explains that it is of great importance that the CRT in a color monitor be protected from the effects of external magnetic fields, and, in particular, time-varying magnetic fields. However, this method provides no compensation to the frequency- dependent amplitude and phase responses of the sensor that picks up incident electromagnetic radiation and the device which generates the cancellation radiation. RECTIFIED SHEET (RULE 91) ISA/EP Likewise, in U.S. Pat No. 5,132,618, Sugimoto shows a magnetic resonance imaging system that includes active shield gradient coils for magnetically canceling leakage fields that would otherwise produce eddy currents in the heat shield tube. A common method for providing shielding to an electromagnetic pickup is to utilize identical pickup coils connected in series or in parallel so as to cancel the effects of uniform electromagnetic radiation. Pizzarello shows such a system in U.S. Pat No. 5,045,784 for reducing inductive noise in a tachometer coil. An electric tachometer is a coil of wire that may be attached to a moving part of a motor that passes through a stationary magnetic field. The motion of the wire through the magnetic field induces a voltage that is indicative of the motor's speed. However, if the motor is powered by electricity, changes in the current powering the motor will cause a magnetic flux, which will also produce a voltage in the coil. Pizzarello shows a stationary pickup coil that is responsive to magnetic flux, and a means for subtracting the pickup voltage from the tachometer voltage. Likewise, in U.S. Pat No. 4,901,015, Pospischil shows a cancellation circuit for canceling the response of a magnetic pickup generators to ambient electromagnetic fields. Pickups used in integrated drive generators are responsive to ambient electromagnetic fields produced by the generator. A first magnetic pickup responsive to the ambient magnetic field and a modulated flux field produced by a rotating shaft is combined with a second magnetic pickup responsive to the ambient electromagnetic fields in order to cancel the inductive effects of the ambient electromagnetic field. Such pickup assemblies are also used with electric guitars and are known as "hum-bucking" pickups. This technique is not effective for providing a high degree of cancellation because slight differences between the pickups, even pickups that are substantially identical, cause the frequency-dependent amplitude and phase response of the pickups to differ significantly from each other. Thus the pickup signals will not be exactly out of phase and equal in amplitude when they are combined. A prior-art method for providing shielding to an electromagnetic pickup from an electromagnetic source that produces a non-uniform field is to "unbalance" either the pickup device or the electromagnetic source. Such a method is described by Hoover in U.S. Pat No.4,941,388. Hoover uses amplitude-adjustment techniques to compensate for amplitude variations between the responses of separate pickups to electromagnetic radiation generated by an electromagnetic sustaining device for driving the vibrations of a string on an electric guitar. However, Hoover does not compensate for differences in the pickup coils which cause the amplitude-variation of the responses of the pickups to be frequency-dependent, thus resulting in poor cancellation over a broad range of frequency. Furthermore, Hoover does not compensate for phase-variations that occur between different pickup coils. The resulting cancellation from the unbalancing method is poor. Hoover describes the operation of negative feedback in a system where a magnetic pickup provides an electrical signal to a magnetic driver which generates a magnetic field to which the pickup responds. Hoover mentions that the system tends to drift from the negative feedback condition at higher frequencies, and identifies the cause of this drift as distortions in the phase-response of the system resulting from the pickup, driver and amplifier in the system. Hoover does not present an effective method for controlling the phase-response of the system, nor he does not present the mathematical relationships between phase and frequency resulting from the driver and pickup coils. Rather, Hoover proposes the use of a low-pass filter to reduce the gain of the system at which the negative feedback condition breaks down. Methods of active phase-compensation are described by Rose in U.S. Pat No. 4,907,483, U.S. Pat No. 5,123,324, and U.S. Pat No. 5,233,123. Rose uses active circuits for determining the frequency or frequency range of an electrical signal from an electromagnetic pickup. Active phase-adjustment is applied to the pickup signal, which is used to power an electromagnetic driver that generates an electromagnetic driving force on a vibratory ferromagnetic element of a musical instrument The purpose of the phase-adjustment of the pickup signal is to provide a driving force to the vibratory element that is substantially in-phase with its natural motion. Because the purpose of Rose's invention is to improve the efficiency of the electromagnetic drive force on the element it is apparent that a passive phase-compensation circuit would be preferable to Rose's active phase-compensation circuit However, Rose does not realize the mathematical relationships between phase and frequency which provide the basis for constructing a passive phase-compensation network. Furthermore, Rose's invention does not provide simultaneous phase-compensation to more than one harmonic. Another method for providing electromagnetic shielding is to orient the angle of a pickup coil to incident electromagnetic radiation such that the electrical current induced in the coil by the electromagnetic radiation will substantially cancel. One application of this method is shown by Burke in the Handbook of Magnetic Phenomena, published in 1986. Burke uses a transmitting coil that produces electromagnetic radiation and a receive coil which senses radiation. The two coils can be configured in such a way that no energy is transferred between the transmitting and receiving coils. Burke shows the receiving coil oriented with the axis of its turns at right angles to the direction of the magnetic field produced by the transmitting coil. Burke explains that the instantaneous generated voltage of the receive coil is determined by the instantaneous rate of change of the magnetic flux passing through the coil. If the flux is directed at right angles to the coil's axis, none of it is intercepted by the coil, and the instantaneous rate of change through the coil is zero. This method of cancellation was used in an electromagnetic sustain device for electric guitars marketed by T Tauri Research of Wilmette Illinois in November, 1988, and patented by Tumura, European Patent AppUcation No. 92307423.1 filed on August 13, 1992, and U.S. Pat No. 5,292,999. The actual effectiveness of this technique is limited by several factors, such as the uniformity of the pickup coil's windings, the uniformity of the electromagnetic radiation near the pickup, interference due to other nearby conducting materials, and the difficulty of precisely positioning a pickup coil in a field whose intensity varies as the inverse square of the distance from its source. Another method for providing active electromagnetic shielding is the differential transformer also shown by Burke. The differential transformer comprises a drive coil for generating a magnetic flux, and two pickup coils wrapped around a ferromagnetic core that includes a moveable armature that, when moved, varies the reluctance of the magnetic path associated with each pickup coil. If the two pickup coils are identical, and if the two magnetic paths about which they are wound are identical, the voltages induced in each pickup coil will be the same. However, Burke explains that the two pickup coils nor the two magnetic paths can be made exactly the same, therefore a differential transformer will always have some output voltage under zero stimulus. Coils of wire whose currents support magnetic fields in space function as antennas radiating electromagnetic energy. There are several cancellation methods used with antennas that act as electromagnetic shielding. One of these methods is the basis of operation for a sidelobe canceller which uses an auxiliary antenna in addition to a main antenna. Combining the outputs from the two antennas results in cancellation of the antenna beam pattern in the direction of a noise source so that the effective gain of the antenna in that direction is very small. Likewise, the multiple sidelobe canceller addresses the problem of multiple noise sources. Delay line cancellers are used in systems where multiple radar pulses are transmitted, and are used to detect moving objects. In a single-element delay line canceller, a received pulse is delayed and added to another pulse received later so that the pulses reflected by stationary objects are out of phase and thus cancel, whereas the pulses reflected by moving objects do not cancel. Several methods are used to allow an antenna to simultaneously transmit and receive electromagnetic radiation. For example, in a continuous wavelength radar system, a single antenna may be employed since the necessary isolation between transmitted and received signals is achieved via separation in frequency as a result of the Doppler effect The received signal enters the radar via the antenna and is heterodyned in a mixer with a portion of the transmitted signal to produce a Doppler beat frequency. An intermediate-frequency receiver may use separate antennas for transmission and reception. A portion of the transmitted signal is mixed with an intermediate frequency, then a narrow-band filter selects one of the sidebands as the reference signal which is mixed with the signal from the receiver antenna. It is one object of the present invention to provide active electromagnetic shielding for canceling the effects of electromagnetic induction in electrical circuits. It is a related object of the present invention to reduce interference between transmitters and receivers of electromagnetic radiation that operate simultaneously. It is another object of the present invention to provide a cancellation circuit that allows a single antenna element to simultaneously transmit and receive electromagnetic radiation. It is still another object of the present invention to compensate for frequency-dependent amplitude and phase responses of electromagnetic receivers and transmitters. SUMMARY OF THE INVENTION In accordance with the present invention, a cancellation circuit is provided for canceling the inductive effects of electromagnetic radiation. The cancellation circuit comprises a means for acquiring or generating an electrical reference signal that is similar in shape to the inductive electrical signal produced by the electromagnetic radiation, an amplitude-adjustment circuit that adjusts the amplitude of either or both the reference signal and an electrical pickup signal containing an inductive noise component, a phase-adjustment circuit that adjusts the relative phase between the reference signal and the pickup signal such that when these signals are combined, the inductive noise component will be canceled, and a combining circuit that combines the reference and pickup signals to produce a pickup signal that is substantially free from inductive noise. In one aspect of the present invention, the reference signal is obtained from an electromagnetic pickup that is responsive to external magnetic flux. In another aspect of the present invention, the reference signal is obtained from part of the electrical signal that is used to generate the external magnetic flux. In still another aspect of the present invention, the reference signal is generated by a signal generator. The present invention provides substantial electromagnetic shielding capabilities compared to prior-art shielding devices. Because the present invention actively shields from electromagnetic flux, it is non-intrusive compared to passive shielding technologies which require complete enclosure in order to provide optimum shielding and use materials that are heavy and bulky. Thus the present invention may be used in order to reduce or eliminate the need for passive electromagnetic shielding in certain applications. Furthermore, in addition to being superior for shielding electromagnetic radiation compared to prior-art active electromagnetic shielding technologies, the present invention may be adapted to prior-art shielding devices to improve their performance. The cancellation effect of the present invention allows electromagnetic pickups to operate in environments containing high levels of electromagnetic noise. For example, the present invention may be integrated into a sustaining device for a stringed musical instrument as described by Rose and Hoover, and provide a very small sustain device that both picks up and drives the vibrations of a string on the musical instrument This sustain device would be much smaller than the devices shown by either Rose or Hoover because the improved shielding capability of the present invention allows for the electromagnetic pickups, which pick up string vibrations, and the driver, which generates an electromagnetic flux to drive those vibrations to be placed very close to each other, or even share the same structure without the effects of electromagnetic interference. Other applications of the present invention include electric tachometers that operate near devices that generate large amounts of magnetic flux, and other electromagnetic receivers such as radars that operate near sources of electromagnetic radiation. This aspect of the present invention allows an electromagnetic antenna to simultaneously operate as a transmitter and receiver by decoupling the receiver- response to the transmitted signal. The present invention may also be used to cancel the response of a radar to ground clutter. Another aspect of the present invention further includes a compensation circuit for adjusting the pickup signal's amplitude and or phase in order to compensate for frequency-dependent amplitude and phase response of the pickup. The compensation circuit may also compensate for frequency-dependent amplitude and or phase variations of electromagnetic flux generated by an electromagnetic flux generator, such as a drive coil. The present invention may be integrated into a prior-art active magnetic shielding circuit that generates a canceling magnetic flux for canceling external magnetic flux. The present invention would provide a more accurate response to external magnetic flux, and thereby improve the cancellation effect of the circuit Such a circuit may be used to provide active electromagnetic shielding to instruments that are sensitive to magnetic or electromagnetic fields, and have applications as shielding devices for atomic clocks, magnetic resonance imaging apparatus, tactical instrumentation, cathode ray tubes, satellites, and spacecraft In another embodiment of the present invention, the electromagnetic flux generated by the drive coil provides a magnetic force upon a moving ferromagnetic element The phase of the electromagnetic flux generated by the system may be adjusted to provide electromagnetic damping to the ferromagnetic element, and thus act as a stabilizer for that element The electromagnetic flux may be adjusted in phase to drive the oscillations of the ferromagnetic element, as discussed by Rose, except that a broad range of driving frequencies may be compensated, thus allowing for the driving of the harmonics as well as the fundamental frequency of the element These and other aspects of the present invention will become apparent to those skilled in the art upon consideration of the following detailed descriptions of the preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a prior-art cancellation circuit FIG. 2 is a schematic view of a cancellation circuit of the present invention. FIG. 3A is a schematic view of a phase-adjustment circuit that may be used in the cancellation circuit of the present invention. FIG. 3B is a schematic view of another phase-adjustment circuit that may be used in the cancellation circuit of the present invention. FIG. 3C is a schematic view of a circuit of another embodiment of the present invention. FIG. 4A is a schematic view of a cancellation circuit of the present invention that illustrates another method of phase-adjustment that may be utilized. FIG.4B is a schematic view of another phase-adjustment circuit that may be used in the cancellation circuit of the present invention. FIG. 4C is a schematic view of another phase-adjustment circuit that may be used in the cancellation circuit of the present invention. FIG. 5 is a plot of cancellation relative to signal frequency for three different cancellation circuits. FIG. 6 is a schematic view of a cancellation circuit of the present invention that generates an electromagnetic flux in response to a pickup signal, and includes a compensation circuit for compensating for frequency-dependent phase and or amplitude variations in electrical signals used to generate the electromagnetic flux. FIG. 7 is a schematic view of a cancellation circuit of the present invention that includes a compensation circuit and is used to provide an electromagnetic drive force to a ferromagnetic element FIG. 8A is a schematic view of a compensation circuit of the present invention. FIG. 8B is a schematic view of another compensation circuit of the present invention. FIG. 8C is a schematic view of another compensation circuit of the present invention. FIG. 9 is a schematic view of another cancellation circuit of the present invention wherein cancellation of incident electromagnetic flux is achieved by generating an out- of-phase electromagnetic flux. FIG. 10 is a schematic view of a cancellation circuit of the present invention wherein a pickup coil and the drive coil are wrapped around the same core. FIG. 11 is a schematic view of a cancellation circuit of the present invention wherein the pickup and drive coils are wrapped around the same core. FIG. 12 is a schematic view of a cancellation circuit of the present invention wherein the reference signal is obtained from a signal generator used to provide a drive signal that generates the electromagnetic flux. FIG. 13 is a schematic view of a cancellation circuit of the present invention wherein the reference signal is obtained from splitting the drive signal used to generate an electromagnetic flux. FIG. 14A is a schematic view of a cancellation circuit of the present invention for a single-element transmit/ receive system that includes a harmonic compensation circuit for canceling the non-linear response of nearby permeable materials. FIG. 14B is a schematic for a harmonic compensation circuit of the present invention. FIG. 14C is a schematic for another harmonic compensation circuit of the present invention. FIG. 14D is a schematic for another harmonic compensation circuit of the present invention. FIG. 15 is a schematic view of another cancellation circuit of the present invention for a single-element transmit/ receive system. FIG. 16 is a schematic view of a cancellation circuit of the present invention used in a device that compensates for magnetic fields. FIG. 17 is a schematic view of another cancellation circuit of the present invention used in a device that compensates for magnetic fields. FIG. 18 is a schematic view of a cancellation circuit of the present invention for another single-element transmit/ receive system. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A prior art balancing device for a pair of electromagnetic pickups is shown in FIG. 1 as including a two coil assemblies, 10 and 12, two amplifiers, 14 and 16, and a combining means 18. The first pickup coil 10 has values of resistance and inductance of Ri and Li, respectively. The second pickup coil has values of resistance and inductance of R 2 and L_, respectively. The pickup coil 10 is connected to the input of the amplifier 14 and the pickup coil 12 is connected to the input of the other amplifier 16. The inputs to the amplifiers 14 and 16 each have a capacitor C j and C-, respectively, connected to electrical ground as is commonly done to filter out high-frequency and white noise from the pickup signals. The outputs of the amplifiers 14 and 16 are combined by a combining circuit 18, which may be a voltage divider, a summing amplifier, or a differential amplifier. The pickup coils 10 and 12 are responsive to the time-rate-of change of the flux of magnetic induction, also referred to as "electromotive force," and herein, for simplicity, referred to as "magnetic flux." A magnetic flux is referred to as an "external" magnetic flux if it is produced by a source external to the pickup coils 10 and 12. External magnetic flux induces a first electrical pickup signal in pickup coil 10 and a second electrical pickup signal in pickup coil 12. Due to coil-positioning with respect to the external magnetic flux, coil properties, and properties of materials (not shown) which the coils 10 and 12 may surround, the ampUtude of the first electrical pickup signal will most likely differ from the ampUtude of the second electrical pickup signal. Thus amplifiers 14 and 16 may be used to change the ampUtude of either or both of the first and second electrical pickup signals. If the pickup signals are out of phase, the combining circuit 18 is a voltage divider or a summing amplifier. If the signals are in phase, then the combining circuit 18 is a differential amplifier. However, the relative phase between the first and second electrical signals will tend to be substantiaUy different than 0 or 180 degrees, thus providing poor cancellation of the signals at the output of the combining circuit 18. The impedance Z 1 of the first pickup coil 10 is related to the coil's 10 resistance Ri and inductance L t : Z 1 = R- + i-oL^ Likewise, the impedance Z 2 of the second pickup coil 12 has the value: Z 2 = R- + iωL-, where ω represents the frequency of the pickup signals multipUed by 2 Pi. The voltage V lin at the input of the first amplifier 14 is ^ - V- [(1 - co'QL.) - iωQRJ/KlV L-) 2 + ω^R- 2 ], where V x is the voltage induced in the coil 10 by external magnetic flux. The voltage V^ of the second pickup signal is ^ - V 2 [(1 - ω'C-L-) - iωC-RJ/KlV L-) 2 + ω^ - 2 ], where V 2 is the voltage induced in the coil 12 by external magnetic flux. IncidentaUy, V α and V 2 are proportional to the magnitude of external magnetic flux at the locus of each pickup coil 10 and 12. The gain imparted to one or both pickup signal voltages V τ and V 2 by the amplifiers 14 and 16 will correct for differences in the ampUtude between Vi and V 2 but wiU not correct for phase differences between those signals. The phase of the first pickup signal voltage V lin has the value 0- = -ArcTan (ωC.R 1 /(l-ω 2 C 1 l_ 1 )), and the phase of the second pickup signal voltage V-^ has the value 0 2 = -ArcTan (ωC-R-/ (l i^). It is typical for the values of I_- and R x to differ substantially from the values of l^ and R y respectively, even for pickup coils having identical numbers of windings. For example, two coils of 34 gauge copper wire, each wound 330 times around identical cores yielded values of resistance of 16.5 and 16.7 ohms, and values of inductance of 205 uH and 194 uH, respectively. Thus when only the amplitudes of the two signals are adjusted so that they are equivalent, the relative phase between the signals prevents optimal cancellation of the signals. The expressions shown for the pickup voltages V__n and V___n are very accurate, but not exact representations for illustrating the differences in the phase variations between the pickup coils 10 and 12. The exact impedance relations for the pickup coils 10 and 12 should also include capacitive effects. Other factors that may contribute to phase variations between the signals produced by the pickup coils 10 and 12 include ground oscillations, complexities resulting from the fact that each of the pickup coils 10 and 12 acts as a source for the electrical pickup signals, possible electrical loading between the two pickup coils 10 and 12, and variations in how the voltage leads the current in the coils 10 and 12 resulting from inductance and capacitance in each of the coils 10 and 12. Although the expressions shown for the pickup voltages Vim and V_in do not provide exact representations for the differences in the phase between the pickup coils 10 and 12, these expressions are accurate to a great degree, and represent the basis from which extremely effective canceUation circuits can be designed. It wiU be appreciated that even more precise representations of the electrical signals induced in magnetic pickups wiU enable the design of canceUation circuits that are even more effective. An embodiment for a canceUation circuit of the present invention is shown in FIG. 2 as including a pair of pickup coils 20 and 22, a pair of ampUtude-adjustment circuits 24 and 26, a phase-adjustment circuit 25, and a combining circuit 28. The pickup coil 20 is connected to a phase-adjustment circuit 25, and the output of the phase- adjustment circuit 25 is connected to the input of an ampUtude-adjustment circuit 24. The pickup coil 22 is connected to the input of an ampUtude-adjustment circuit 26. The outputs of both ampUtude-adjustment circuits, 24 and 26, are connected to a combining circuit 28. The output of the combining circuit 28 provides an electrical signal that is substantiaUy free from the effects of electrical noise caused by the response of the pickup coils 20 and 22 to external magnetic flux. The pickup coils 20 and 22 are responsive to external magnetic flux, which induces a first signal voltage V 1 at the output of the first pickup coil 20, and a second signal voltage V 2 at the output of the second pickup coil 22. The phase of signal voltage V j wiU be 0 α and the phase of signal voltage V 2 will be 0 2 . Because both 0 X and 0 2 are functions of signal frequency ω, we wiU write 0 x (ω) and 0 2 (ω). The pickup coil 20 is connected to the input of a phase-adjustment circuit 25 which provides a phase-shift F(ω) = (0 2 (ω) - 0 : (ω)) to V. in order to compensate for the phase-difference between signals V x and V 2 . The nature of the phase-adjustment circuit 25 is determined by the frequency-range of signal-cancellation required. The output of the phase-adjustment circuit 25 is connected to the input of an ampUtude-adjustment circuit 24. The pickup coil 22 is connected to an ampUtude-adjustment circuit 26. Both ampUtude-adjustment circuits, 24 and 26, may provide ampUtude-adjustment to the pickup signals V t and V 2 , respectively, or only one of the ampUtude-adjustment circuits 24 or 26 may provide ampUtude adjustment while the other circuit 24 or 26 acts only as a buffer. Because phase-adjustment circuits, such as phase-adjustment circuit 25, typicaUy change signal- ampUtude as weU as phase, it is preferable that the ampUtude-adjustment circuits, 24 and 26, have Uttle effect on signal-phase. Thus the ampUtude-adjustment circuits, 24 and 26, may comprise non-inverting amplifiers. The outputs of the ampUtude- adjustment circuits, 24 and 26, are combined in the combining circuit 28 in order to cancel the effects of external magnetic flux picked up by coils 20 and 22. Depending on whether the output signals of the ampUtude-adjustment circuits, 24 and 26, are in-phase or out of phase, the combining circuit 28 may comprise a voltage divider, a summing amplifier, or a differential amplifier. It wiU be appreciated that the coils 20 and 22 may be wrapped around a core (not shown) such as a core comprising a ferromagnetic material. It will also be appreciated that one or more additional phase-adjustment circuits may be included in series with coil 20 and or coil 22. Furthermore, it wiU be appreciated that ampUtude-adjustment circuits such as ampUtude-adjustment circuit 24 may precede phase-adjustment circuits, such as phase-adjustment circuit 25. Several phase-adjustment circuits are shown in FIG. 3 which may be used in the circuit shown in FIG. 2. The circuit in FIG. 3A is commonly referred to as an "aU-pass filter," and provides a phase-shift of 0 = 180° - 2 ArcTan (ω R^ C g ) while producing Uttle ampUtude-variation with respect to signal frequency. The circuit shown in FIG. 3B is also an aU-pass filter, and produces a phase-shift of 0 = 2 ArcTan (ω R 8 C 4 ). The aU- pass filters in FIG. 3A and 3B may be preceded by a buffer amplifier (not shown). It is sometimes desirable to have substantial noise-canceUation over only a narrow frequency-range. This is caUed "notch-canceUation" and may be used in a single-frequency or band-limited system. One appUcation for notch-canceUation is when an external magnetic flux contains a weak signal having a significantly different frequency than the noise that accompanies it then a canceUation circuit which cancels a narrow frequency range including the noise, but not the desired signal would be preferable. Ferromagnetic materials used for pickup cores have the property of non¬ linear responsiveness to magnetic flux, which can be observed in the pickup signal of a coil as exhibiting a higher harmonic of the frequency of the magnetic flux. In order to observe the extent of the core material's non-linearity, it is preferable to cancel only the primary pickup signal, which has the same frequency ω as the appUed magnetic flux. Typically, the higher harmonic signatures caused by a core's non-linearity is at least several orders of magnitude less than the intensity of the primary signal induced in the coil. Thus the method of notch-canceUation provides an advantage over conventional electrical filtering techniques in both simpUcity and performance. FIG. 3C shows another embodiment for a cancellation circuit of the present invention. Two pickup coils 30 and 32 are each connected to a phase-adjustment circuit 35 and 37, respectively. The phase-adjustment circuits 35 and 37 are each connected to the input of an ampUtude-adjustment circuit 34 and 36, respectively. The outputs of the ampUtude-adjustment circuits 34 and 36 are combined by a combining circuit 38. The output of the combining circuit 38 is provided that is substantiaUy free from inductive noise. The phase-adjustment circuits shown in FIG. 3A and 3B may be used as the phase-adjustment circuits 35 and 37 shown in FIG. 3C. Furthermore, the phase- adjustment circuits shown in FIG. 3A and 3B include a means for adjusting the ampUtude of electrical signals through adjustment of the resistors Rg and II- in FIG. 3A, and resistors R, and R 10 in FIG. 3B. _ . For the case of notch-canceUation in which the relative phase between the pickup signals from the first and second pickup coils 30 and 32 is very smaU, such as when the coils 30 and 32 are very close to being identical or when a phase-adjustment circuit (not shown) has already created this condition, then it is preferable to select types of phase- adjustment circuits 35 and 37 that wiU cause a very narrow frequency-range in which the canceUation is substantial. If the pickup signals from the two pickup coils 30 and 32 are in phase, this may be accompUshed by selecting the all-pass filter shown in FIG 3A as one phase-adjustment circuit 35 and selecting the aU-pass filter shown in FIG 3B as the other phase-adjustment circuit 37. This selection is suggested because as the signal- frequency ω changes, the phase of the pickup signal of one of the phase-adjustment circuits 35 wiU increase while the phase of the pickup signal of the other phase- adjustment circuit 37 wiU decrease, thus causing a rapid change in the relative phase with respect to frequency ω. To maximize the change in the relative phase near the "notch frequency" where the canceUation is most substantial, one could select values of Rg and in FIG. 3A and values of R g and C 4 in FIG. 3B such that and ω_RgC 4 are nearly equal to 1 for the notch frequency ω n . To further narrow the canceUation notch about the notch frequency ω„, the phase-adjustment circuits 35 and 37 may each include multiple aU-pass filters as shown in FIG. 3A and FIG. 3B. For the case in which canceUation is desired over a broad frequency range, such as when the pickups 30 and 32 are part of a feedback circuit that is prone to osciUate, making it necessary to cancel the higher harmonic terms that wiU accompany the primary signal, then phase-adjustment circuits 35 and 37 may be selected to broaden the canceUation noteh about the notch frequency ω n . For example, the choice of phase-adjustment circuits 35 and 37 for two pickup signals that are in phase may both be of the type of aU-pass filters shown in FIG. 3A or FIG. 3B where the values of resistance Rg or Rg and capacitance or C 4 are chosen to minimize the relative phase and ampUtude variations with respect to frequency between the two pickup signals. FIG. 4A shows how two phase circuits may be integrated into a circuit comprising two pickup coils 40 and 42. CoU 40 is connected to series element 45 which may include resistors and or inductors (not shown) connected in series with the coil 40. Likewise, coU 42 is connected to series element 47 which may include resistors and or inductors (not shown) connected in series with the coU 42. Series element 45 is connected to the input A of an ampUtude-adjustment circuit 44 and series element 47 is connected to the input B of an ampUtude-adjustment circuit 46. Input A includes a resistor Rg connected to electrical ground, and input B includes a resistor R 4 connected to electrical ground. Together, the series element 45 and resistor R 3 form one phase- adjustment circuit and the series element 47 and the resistor R 4 form another phase- adjustment circuit The outputs of the ampUtude-adjustment circuits 44 and 46 are connected to the input of a combining circuit 48 which combines the output signals of the ampUtude-adjustment circuits such that the noise-signal caused by external magnetic flux substantiaUy cancels. The effective impedance of the coU 40 at the input A of the amplitude-adjustment circuit 44 includes the actual impedance of the coil 40 added to the impedance of the series element 45, and is represented by Z x = R x + iωL The effective impedance of the coU 42 at the input B of the amplitude-adjustment circuit 46 includes the actual impedance of the coil 42 added to the impedance of the series element 47, and is represented by Z 2 = R 2 + ioL j . The voltage of the pickup signal induced in the coil 40 by external magnetic flux having frequency ω, measured at the input A is V 1A = R 3 V 1 / (R 1 + R 3 + iωL 1 ), where V x is the voltage-magnitude of the signal induced in the pickup coU 40 by the external magnetic flux. The signal voltage induced in the coil 40 by external magnetic flux having frequency ω, measured at the input B is V 2B = R 4 V 2 / (R 2 + R 4 + iωL 2 ), where V 2 is the voltage-magnitude of the signal induced in the pickup coil 32 by the external magnetic flux. The phase of the voltage of the pickup signal at the input A is 0- = ArcTan(-ωL 1 /(R 3 +R 1 )), and the phase of the voltage of the pickup signal at the input A is 0 2 = ArcTanf-ω ^+R-))- In order that 0 α = 0 2 for a broad range of signal-frequencies, ω, it is necessary that the series element 45 and or series element 47 be adjusted such that L j /^+R.) = I^/^+R-). This may also be accompUshed by adjusting resistors R 3 and or R 4 . However, if we look at the equations for signal voltage at the inputs A and B of the ampUtude-adjustment circuits 44 and 46, respectively, we note that equivalence of the ratios just discussed does not, by itself, provide the condition whereby the magnitude of the voltage difference V 1A - V 2B remains substantiaUy constant as ω changes. Thus in order to assure optimal canceUation over a broad range of signal frequencies, it is necessary that the series elements 45 and 47 be adjusted such that the effective resistances R x and R 2 are equivalent and the effective inductances I^ and L- are equivalent It is also necessary that resistance R g equal resistance R 4 . It is possible to replace resistors R 3 and R 4 with capacitors (not shown) for filtering out high-frequency noise. However, for optimal canceUation over a broad range of signal-frequencies, it is necessary that both of these capacitors (not shown) have substantiaUy equal values. It wiU be appreciated from the equations representing the voltages V IA and V_B at the amplifier inputs A and B, respectively, that the series elements 45 and 47 may each include a large value of series resistance so as to increase the effective resistances R_ and R 2 of the pickup coUs 40 and 42, respectively. This will reduce the frequency-dependent ampUtude and phase variations of the pickup signals V IA and V 2B . However, it is preferable that the increase in the effective resistances Ri and R 2 of the pickup coils 40 and 42, respectively, not be the only means of phase-adjustment used in the circuit as other phase effects that are unrelated to the signal-voltage equations for V IA and V 2B tend to occur. Consider the circuit shown in FIG.4A in the case in which it is not optimized for canceling the effects of external magnetic flux. The phase-shift between voltages V 1A and V 2B required to match their phases is F(ω) = ArcTan(-ωL 1 /(R 3 +R 1 )) - ArcTan -ωL-/ R 4 +R-)). A phase-adjustment circuit that provides the required phase-shift may include a buffered input and precede either or both ampUtude-adjustment circuits 44 and 46, or may foUow either or both amplitude-adjustment circuits 44 and 46. The components of this phase-adjustment circuit are shown in FIG. 4B and FIG. 4C. The phase-adjustment circuit shown in FIG. 4B is a passive lead network. An input voltage V^ is appUed across terminals C and G. The output voltage V^ of this circuit is measured across terminals D and G. The output voltage is V mt = ta R 12 (iωC , 0 R n + lJΛR^iωQΛ- + 1) + R π ). If R n » R 12 , then the phase-shift 0 = ArcTan ((-R^C-,,). The phase-adjustment circuit shown in FIG.4C is a passive lag network. An input voltage Vin is applied across terminals E and G. The output voltage V out of the circuit is measured across terminals F and G. The output voltage is V^ = V,- R 14 / ((R 14 + R 13 ) + iω 5 R 14 R 13 ). If R 14 » R 13 , then the phase-shift 0 = -ArcTan (coR^s). The circuits in FIG. 4B and FIG. 4C provide the basis for constructing F(ω): One can use the passive lead network shown in FIG.4B in series with the passive lead network shown in FIG. 4C, utilizing appropriate buffering between the lead and lag networks, such as a buffered amplifier (not shown), and selecting R 14 and C-g such that R 14 Cis = L./(R 3 + Rj) and selecting R u and C- 0 such that R--C.,, = I (R 4 + **_)• Another canceUation circuit is shown in FIG. 4D, and includes a first pickup coil 50 wrapped around a first core 51 in a predetermined direction, a second pickup coil 52 wrapped in the opposite direction around a second core 53 and connected in series with the first pickup coil 50, and a positioning device 55 for adjusting the position or orientation of the second core 53. The first pickup coil 50 has resistance Ri and inductance Li. The second pickup coil 52 has resistance R 2 and inductance L 2 . The impedance Zc of the pickup coils 50 and 52 at the output of the second pickup coil 52 is Zc = Ri + R2 + iω (Li + L 2 ). The voltage induced in each pickup coU, 50 and 52, by magnetic flux wiU tend to be substantiaUy 180 degrees out of phase as the pickup coils 50 and 52 are connected in series, which aUows opposite signals to be induced in the combined pickup device with substantiaUy identical frequency-dependent ampUtude and phase relationships. AmpUtude-adjustment is provided by the positioning device 55 which translates and or rotates the second core 53 relative to equipotential lines of magnetic flux (not shown) which induce electrical pickup signals in the second pickup coil 52. The positioning device 55 may move the second core 53 closer to or farther away from the source of magnetic flux (not shown). Likewise the positioning device 55 may tilt the second core 53 so as to adjust the ampUtude of the electrical signal induced in the pickup coil 5Z Similarly, the positioning device 55 may adjust the position of the second core 53 relative to the second pickup coil 52 so as to adjust the magnitude of magnetic flux inside the pickup coil 52, which adjusts the ampUtude of the electrical signal induced in the pickup coU 52. It wiU also be appreciated that the positioning device 55 may be used to move the source of magnetic flux (not shown) for adjusting the ampUtude of the electrical pickup signals induced in the first and second pickup coils, 50 and 52. Thus it is apparent that there are several ways to provide ampUtude- adjustment by adjusting the relative positions between a pickup core and a source of magnetic flux. However, due to the inverse-square relation of the intensity of magnetic flux with respect to distance from the source of magnetic flux, it is necessary to provide a precise degree of position-adjustment in order to obtain a significant degree of canceUation of the induced pickup signals. For this reason, this method of ampUtude- adjustment is not as preferable as the methods which utilize electrical gain or attenuation for ampUtude control. It should be appreciated that the phase-adjustment circuits shown in FIG. 3 and FIG. 4 are only a few of the many phase-adjustment circuits that can be used in a cancellation circuit for substantially eliminating electrical noise in a pickup signal caused by external magnetic flux. The phase-adjustment circuits shown, as weU as other phase-adjustment circuits may be used in combination for either broadening or narrowing the frequency range where a high degree of signal canceUation occurs. Phase-adjustment circuits may also be used for adjusting ampUtudes of electrical signals, and thus may be employed as combined phase and ampUtude-adjustment circuits. A graphical analysis of different types of canceUation is shown in FIG. 5. This graph plots canceUation in decibels and represents the combined output of two pickups at the output of a combining circuit, such as the voltage-magnitude Vout of combining circuit 48 shown in FIG. 4A, divided by the voltage-magnitude of the pickup signal at one of the inputs to the combining circuit 48. In this graph, canceUation is plotted against signal frequency (Hz). Plot 1 of FIG. 5 represents canceUation obtained by a circuit that does not compensate for phase-differences between two pickup signals, such as the prior art circuit shown in FIG. 1. Plot 2 of FIG. 5 represents "notch-canceUation" as explained above with reference to the canceUation circuit shown in FIG. 3C. The frequency at which the notch occurs can be changed by adjusting the phase-adjustment circuits 35 and 37. Plot 3 of FIG. 5 represents canceUation obtained by a canceUation circuit such as the canceUation circuit shown in FIG. 4A, that provides substantial canceUation of external magnetic flux over a relatively broad range of frequency. This curve iUustrates a very broad notch centered at a notch frequency ω_. At frequencies below and above the notch frequency ω n the degree of cancellation begins to deteriorate. In the case where one uses a canceUation circuit such as the one shown in FIG.4A, it is possible to improve the canceUation at frequencies below the notch frequency ω n by adjusting the resistance in either or both series elements 45 and 47 so that R α better approximates the value of R^ Thus Z r better approximates Z 2 at low frequencies. It is also possible to improve the canceUation at frequencies above the notch frequency ω n by adjusting the inductance in either or both series elements 45 and 47 so that L_ better approximates the value of I^. Thus Z x better approximates Z 2 at high frequencies. Furthermore, the overaU level of canceUation over the entire frequency range may be improved by adjusting the values of resistors Rg and R 4 such that the values of these resistances better approximate each other. It wiU be appreciated that additional phase circuits may be used to provide compensating phase-shifts at low and or high frequencies in order to broaden the notch centered at the notch frequency ω n . FIG. 6 shows a canceUation circuit of the present invention which comprises two pickup coils 60 and 62, phase-adjustment circuits 65 and 67, amplitude-adjustment circuits 64 and 66, a combining circuit 68, a preamplifier 74, a power amplifier 76, and a drive coil 70. The pickup coils 60 and 62 and or the drive coil 70 may also include one or more ferromagnetic cores (not shown). Pickup signals are induced in the pickup coils 60 and 62 by external magnetic flux. In this case, the external magnetic flux is generated by the drive coil 70. The pickup signal from the first pickup 60 is adjusted in phase by the phase-adjustment circuit 65 and the pickup signal from the second pickup 62 is adjusted in phase by the phase-adjustment circuit 67 such that the phases of the two pickup signals are substantiaUy in phase (0 degrees) or out of phase (180 degrees) for a broad range of signal frequencies. The ampUtude of the first pickup signal is adjusted by the amplitude-adjustment circuit 64 and the ampUtude of the second pickup signal may be adjusted by the ampUtude-adjustment circuit 66. However, the ampUtude-adjustment circuit 66 may act only as a buffer and provide no ampUtude-adjustment to the second pickup signal. It will be appreciated that the ampUtude-adjustment circuits 64 and 66 may provide either gain or attenuation to the pickup signals. It will also be appreciated that the ampUtude-adjustment circuits 64 and 66 may be replaced by a means for adjusting the position and or orientation of the pickup coils in order to provide adjustment to the ampUtude of the pickup signals induced in either or both pickup coils 60 and 62. The outputs of both ampUtude-adjustment circuits 64 and 66 are received by a combining circuit 68 that combines the pickup signals such that the pickup signals induced by the external magnetic flux generated by the driver 70 substantiaUy cancel. The output of the combining circuit 68 is amplified by a preamplifier 74. The output of the preamplifier 74 is amplified into a drive signal by a power amplifier 76. The drive signal flows through a drive coil 70 which generates a magnetic flux in response to an external magnetic flux from a source other than the drive coil 70, or a magnetic flux caused by the motion of a ferromagnetic element (not shown) in a magnetic field. For the circuit shown in FIG. 6, it is necessary that a high degree of canceUation be obtained for a broad range of frequencies else the circuit wiU undergo oscillation due to direct magnetic feedback. In general, a circuit wiU osciUate at a frequency at which the feedback gain is positive, that is, when the circuit gain exceeds the circuit losses. If the circuit in FIG. 6 achieves a canceUation profile similar to plot 3 in FIG. 5, then the circuit may be likely to osciUate at a relatively high frequency where the canceUation is not as effective. However, a low-pass filter may be included in the feedback circuit to reduce the feedback gain of the circuit For example, preamplifier 74 may comprise an active low-pass filter. Likewise, one or more high-pass or bandpass filters may be used to eliminate circuit osciUation. It is also possible that the phase- adjustment circuits 65 and 67 and or ampUtude-adjustment circuits 64 and 66 of the canceUation circuit may be designed specificaUy to filter certain frequencies. The phase-adjustment circuits 65 and 67 are designed specificaUy for compensating for frequency-dependent phase-variations between the pickup signals of pickups 60 and 62. However, the phase-adjustment circuits 65 and 67 may provide a specific overaU phase-shift to the combined pickup signal at the output of the combining circuit 68. This overaU phase-shift may compensate for the phase-shift introduced to the drive signal as a result of the frequency responses of the pickups 60 and 62 and the driver 70. The phase-adjustment circuits 65 and 67 may also be used to compensate for phase-shifts in the circuit due to other circuit elements that may precede the drive coU 70. The phase-adjustment circuits 65 and 67, most preferably are preceded by a buffer (not shown), and may precede or follow the ampUtude-adjustment circuits 64 and 66. An embodiment for a canceUation circuit of the present invention is shown in FIG. 7. The circuit in FIG. 7 includes a magnetic element 81, a ferromagnetic core 83, two pickup coUs 80 and 82 wrapped around the core 83, a phase-adjustment circuit 85, the input to the phase-adjustment circuit being connected to electrical ground by a resistor R3. The output of the phase-adjustment circuit is connected to the input of an ampUtude-adjustment circuit 84. The pickup coU 82 is connected to the input of an ampUtude-adjustment circuit 86, this input also includes a resistor R4 connected to electrical ground. The outputs of each ampUtude-adjustment circuit 84 and 86 is connected to a combining circuit 88. The output of the combining circuit 88 is connected to a compensation circuit 89. The compensation circuit 89 is connected to an amplifier 96 which amplifies an input signal into a drive signal which flows through a drive coU 90 wrapped around a ferromagnetic core 90. The pickup coils 80 and 82 are shown approximately equidistant to the drive coil 90, which generates an external magnetic flux F. The pickup coils 80 and 82 receive approximately equal intensities of the external magnetic flux F generated by the drive coil 90. AmpUtude-adjustment of the pickup signals induced in the pickup coUs 80 and 82 by the drive coU's 90 generated external magnetic flux F compensate for differences that may occur between the pickup signals, such as differences in the intensities of the drive coil's 90 generated magnetic flux F at the location of each pickup coil 80 and 82, distortions in the drive coil's 90 generated magnetic flux F resulting from nearby conducting or magnetically permeable materials such as ferromagnetic element 95, and differences in the ampUtude-response between the pickup coils 80 and 82 to magnetic flux including frequency-dependent ampUtude response. Likewise, the pickup signals induced in the pickup coils 80 and 82 by uniform external magnetic flux and magnetic flux generated by other magnetic sources (not shown) disposed in the plane that is the perpendicular bisector of the height dimension of the drive coU 90 wiU also be substantiaUy equal in ampUtude. Thus after phase-shifting by the phase-adjustment circuit 85 and combining by the combining circuit 88, the signals induced in the pickup coils 80 and 82 by uniform external magnetic flux, the magnetic flux F generated by the drive coU 90, and magnetic flux generated by any other magnetic sources (not shown) disposed in the plane that is the perpendicular bisector of the height dimension of the drive coil 90 wiU cancel. The canceUation of the effects of the drive coil's 90 generated magnetic flux F on the combined pickup signal is altered if either a permeable or conducting object enters the space shared by the field patterns F of the drive coU 90 and the pickup coils 80 and 82. If the intruding object is permeable, the field pattern F surrounding the pickup coUs 80 and 82 is distorted, and energy passes directly from the drive coU 90 to the pickup coils 80 and 82 through the distorted field. Thus if the ferromagnetic element 95 vibrates, the frequency of its motion is reproduced in the combined pickup signal. Because of the canceUation of external magnetic flux, the output of the combining circuit 88 wiU comprise only the pickup signals induced by the motion of the ferromagnetic element 95. The compensation circuit 89 provides a phase-shift to the output signal of the combining circuit 88. The output signal of the phase-adjustment circuit 88 is amplified by the amplifier 96 into a drive signal which flows through the drive coil 90 and generates the magnetic flux F. This magnetic flux F may be used to either drive or damp the motion of the ferromagnetic element 95 depending on the phase of the drive signal. It is preferable to provide drive forces to the ferromagnetic element 95 such that the phase-relationship of the drive force to the motion of the ferromagnetic element 95 is not changed by the frequency of the element's 95 motion. This is particularly important if the motion of the ferromagnetic element 95 comprises a pluraUty of different frequencies. For the circuit shown in FIG. 7, the signals from the pickup coils 80 and 82, after being combined, may have a total phase-shift of 0 p = ArcTan( -<DL 2 /( R _ +R _) ) • The phase-shift of the drive signal at the drive coil 90 is 0 d = ArcTan(ωL d /R d ), where R d and L d are the resistance and inductance, respectively, of the drive coil 90. Ignoring any phase-shifts caused by other elements in the circuit the total phase-shift between the magnetic flux F generated by the drive coU 90 and the response of the pickups 80 and 82 to the magnetic flux F generated by the drive coU 90 is 0 t = 0 + 0 d . By adjusting the values of resistances R 2 , R^ and R j , and or the values of inductances 1^ and L__ or combinations thereof, it is possible to cause the ratios ^/(R- i +R . ) and L d /R d be substantiaUy equal, and thus the value of 0, will be substantiaUy zero for a broad range of signal frequencies, ω. The circuits shown in FIG. 8A and FIG. 8B may be used in a canceUation circuit as phase-adjustment circuits, such as phase-adjustment circuit 85, for providing phase- shifts to pickup signals before they are combined, or may be used in the feedback loop of FIG. 7 as a phase-compensation circuit such as phase-compensation circuit 89. The circuit shown in FIG. 8A is a common inverting amphfier. A buffer amplifier 94 may precede the first impedance element Z u . The second impedance element Z 12 provides feedback between the output and inverting input of amplifier 98. The gain resulting from this inverting amplifier is G t = Z 12 / Z n . Thus the ratio Z 12 / Z n may be adjusted to compensate for phase-shifts. The circuit shown in FIG. 8B is a common non-inverting amplifier comprising an amplifier 99 and gain-control impedance values Z 13 and Z 14 . The gain of the non-inverting amplifier is G α = 1 + Z 14 / Z 13 . Likewise, the ratio Z 14 / Z 13 may be adjusted to compensate for phase-shifts. The circuits shown in FIG. 8 also provide a means for compensating for frequency-dependent ampUtude variations in the feedback signal caused by the frequency response of the pickup coUs 80 and 82, the drive coil 90, and any other circuit elements in the feedback loop. For example, the circuit shown in FIG. 7 wiU produce a pickup signal voltage V p = V B R / (B^ + R 4 + iωl^) at the input of amplifier 86, where V B is the voltage induced in the pickup coil 82 by external magnetic flux B. The voltage V B is proportional to the magnitude of the magnetic flux B. The magnitude of magnetic flux B is proportional to the ampUtude of drive current I d in the drive coil 90, where I d = V d / (R d + iωLj) and V D is the drive voltage. Thus the pickup signal voltage V p is proportional to V B R / (R j + R 4 + iωL-) (R d + iωL j ). One can observe from the equation for V p that as ω increases, the pickup signal voltage V wiU decrease. An example of a circuit design that can compensate for the frequency-dependent ampUtude variation of the pickup signal voltage V p is two inverting amplifiers as shown in FIG.8A connected in series. The impedance element Z 12 of the first amplifier 98A comprises a resistor R 12 (not shown) and inductor L 12 (not shown) connected in series. The value of the resistor R 12 is ( R- + R 4 ) and the value of the inductor L^ is L j . The value of the impedance element Z u of the first amplifier 98A is αR^ where α is a scalar constant The impedance element Z 12 of the second amplifier 98B comprises a resistor R^ (not shown) and inductor L^ (not shown) connected in series. The value of the resistor R^ is R d and the value of the inductor L^ is L d . The value of the impedance element Z 21 of the second amplifier is αR 4 . The gain G α of the first amplifier is (1^ + R 4 + ic-L j ) / αR 4 . The gain G 2 of the second amplifier is (R d + iωL j ) / αR 4 . The total gain of this circuit is G t = G J G J , and G t multipUes the pickup voltage V p so that the frequency- dependent nature of the pickup voltage ampUtude is compensated. It should be noted that the values of Ry R^ and R d may be increased to reduce the frequency-dependent effects on the pickup signal voltage V p , however increasing the value of R d substantiaUy reduces the magnitude of magnetic flux generated by the drive coil 90. The frequency-dependent phase-shifts and ampUtude variations that typicaUy occur between a pickup coil and drive coil may be substantiaUy compensated over a broad range of signal frequencies, ω, through the selection process of the values for electrical components in the compensation circuits that adjust both ampUtude and phase-response. TypicaUy, for a feedback system such as shown in FIG. 7, where the motion of a ferromagnetic element is driven by the external magnetic flux generated by the drive coU 90, the frequency-dependence of the phase of the drive signal is of more interest than the frequency-dependence of the amplitude of the drive signal. However, for a feedback system in which a drive coil generates a specific magnetic flux in response to an external magnetic flux, such as for the purpose of canceling an external magnetic flux in a specific region space, controlling both the phase and ampUtude of the drive signal is important Another embodiment for a canceUation circuit of the present invention is shown in FIG. 9. The circuit in FIG. 9 includes a pickup coil 104 wrapped around a pickup core 105. The pickup core 105 may be made of a ferromagnetic material and may be magnetized. The pickup coil 104 is connected to a compensation circuit 106, which connects to a spUtting circuit 108. The spUtting circuit 108 has a first output connected to a first phase-adjustment circuit 109 and a second output connected to a second phase adjustment circuit 110. The first phase-adjustment circuit 109 is connected to the input of a first amplifier 111 and the second phase-adjustment circuit 110 is connected to the input of a second amplifier 112. The output of the first amplifier 111 is connected to a first drive coil 100, and the output of the second amplifier 112 is connected to a second drive coil 102. Both the first and second drive coils 100 and 102, respectively, are wrapped around a drive core 101, and generate a magnetic flux F. The drive core 101 may be made of a ferromagnetic material, and may be magnetized. In this case, the drive core 101 is shaped so that both of its endpoles are in close proximity to a ferromagnetic element 115. The ferromagnetic element 115 induces a current in the pickup coil 104 when its motion disturbs the distribution of magnetic flux F that passes through the pickup coU 104.. The shape of the drive core 101 concentrates the magnetic flux lines F generated by current in the drive coils 100 and 102 so as to provide a more efficient magnetic drive force to the ferromagnetic element 115. An electrical signal V D1 is induced in the pickup coil 104 by magnetic flux generated by the first drive coil 100, and 102, and another pickup signal V κ is induced in the pickup coU 104 by magnetic flux generated by the second drive coU 102. An electrical pickup signal is induced in the pickup coU 104 by magnetic flux produced by other sources such as the ferromagnetic element 115 moving through a static magnetic field. The electrical signals induced in the pickup coil 104 pass through the compensation circuit 106 to the spUtting circuit 108 which spUts the pickup signal into two drive signals. One of the drive signals passes through the first phase- adjustment circuit 109 and is ampUfied by the first amplifier 111. The other drive signal passes through the second phase-adjustment circuit 110 and is ampUfied by the second amplifier 112. The first drive coU 100 has an effective resistance of R D1 and an effective inductance L D1 which results in a total impedance of Z- = R D1 + iωLu-, where ω is the frequency of the drive signal. The second drive coil 102 has an effective resistance of R ∞ and an effective inductance L-^ which results in a total impedance of Z 2 = R^ + iωl-u j . Because the impedances Z and Z 2 of drive coils 100 and 102, respectively, wiU tend to differ from each other, a drive signal flowing through the first drive coU 100 having the same frequency ω as a drive signal flowing through the second drive coU 102 wiU tend to differ in phase and ampUtude from the second drive signal. The phase-adjustment circuits 109 and 110 compensate for frequency-dependent phase differences between the first and second drive signals, and the amplifiers 111 and 112 may provide amplitude-adjustment to either or both of the drive signals in order to compensate for ampUtude differences between the signals. The phase and amphtude- adjustment is performed such that the signals V D1 and V D2 induced in the pickup coil 104 by the first and second drive coils 100 and 102, respectively, are substantially equal in magnitude and 180 degrees out of phase so that they cancel. It wiU be appreciated that the spUtting circuit 108 may be used to adjust the relative magnitudes of the drive signals in the drive coils 100 and 102. It wiU also be appreciated that the phase- adjustment circuits 109 and 110 may be positioned so that they each foUow the amplifiers 111 and 112, respectively. Because phase-adjustment and ampUtude- adjustment need only be applied to one of the two drive signals, this allows for the removal of one of the phase-adjustment circuits 109 or 110. Another method of ampUtude and phase-adjustment involves changing the effective resistance R m and R^ and or the effective inductance L^ and L^ of either or both drive coils 100 and 102. Therefore, the phase-adjustment circuits 109 and 110 foUow the amplifiers 111 and 112, respectively. The phase-adjustment circuits 109 and 110 would comprise resistors (not shown) and or inductors (not shown) connected in series with the drive coUs 100 and 102 so as to adjust their effective resistance R m and Ru 2 and or effective inductance IL^ and L^. In order for the relative phase between the magnetic flux generated by each drive coU 100 and 102 to be substantiaUy 180 degrees, the relationship: ArcTan(ωL D1 /R m ) = ArcTan(ωL D2 /R D2 ) must hold for a wide range of signal frequencies ω. Thus L- J J/R- JJ = L JK /R UJ . However, in order for the relative ampUtudes between the magnetic flux generated by each drive coil 100 and 102 to be substantiaUy equal for a broad range of signal frequencies ω, it is preferable that the effective resistances R D1 = R^ and the effective inductance D1 = L^- It wiU be appreciated that because the effective resistance R D1 and R^ and the effective inductance 1^ and L^ can be adjusted to control both the phase and ampUtude of the drive signals in the drive coUs 100 and 102, then only a single amplifier (not shown) for amplifying a pickup signal into a drive signal is necessary. An alternative canceUation circuit would include a single amplifier circuit (not shown) placed between the compensation circuit 106 and the spUtting circuit 108, and the amplifiers 111 and 112 may be removed. The phase-adjustment circuits 109 and 110 could provide both ampUtude and phase-adjustment to the drive signals going to each drive coU 100 and 102 as described above. An embodiment for a canceUation circuit of the present invention is shown in FIG. 10. The circuit in FIG. 10 includes a first pickup coU 120 wrapped around a pickup core 121, a second pickup coU 122 wrapped around a second core 121, and a drive coU 120 wrapped around the second core 121. The first pickup coU 120 is connected to the input of an ampUtude-adjustment circuit 124. The output of the ampUtude-adjustment circuit 124 is connected to the input of a phase-adjustment circuit 125. The second pickup coU 122 is connected to the input of an ampUtude-adjustment circuit 126. The output of the ampUtude-adjustment circuit 126 is connected to a phase-adjustment circuit 127. The outputs of the phase-adjustment circuits 125 and 127 are connected to a combining circuit 128. The output of the combining circuit 128 is connected to a compensation circuit 129. The compensation circuit 129 is connected to an amplifier 136 which amplifies the input signal into a drive signal which flows through the drive coU 130, and generates a magnetic flux. The pickup coUs 120 and 122 are responsive to the magnetic flux generated by the drive coil 130. However due to the proximity of the second pickup coil 122 to the drive coil 130, the second pickup coil 122 receives a greater magnitude of magnetic flux generated by the drive coU 130 than does the first pickup coil 120. It wiU be appreciated that the second pickup coil 122 may be located inside of the drive coil 130, or that the pickup coils 120 and 122 may be positioned, shielded or otherwise designed such that the second pickup coil 122 receives greater magnetic flux generated by the drive coil 130 than does the first pickup coil 120. The amplitudes of the pickup responses of the first and second pickup coils 120 and 122 induced by the magnetic flux generated by the drive coU 130 are made equivalent by either or both of the ampUtude-adjustment circuits 124 and 126. The phases of the pickup responses of the first and second pickup coUs 120 and 122 induced by the magnetic flux generated by the drive coU 130 are compensated by either or both of the phase-adjustment circuits 125 and 127 so that when the pickup signals are combined in the combining circuit 128, they wiU substantially cancel. However, the response of the pickup coUs 120 and 122 to uniform external magnetic flux will result in a non-zero contribution to the combined signal at the output of the combining circuit 128. The compensation circuit 129 may comprise either or both phase-adjustment and ampUtude-adjustment circuits (not shown) for adjusting the phase and or ampUtude response of the drive signal. The drive signal flows through the drive coil 130 and generates a uniform magnetic flux inside the drive coil 130 that substantiaUy cancels the uniform magnetic flux inside the drive coil 130 generated by other sources (not shown). The core 121 may be a ferromagnetic core, however, ferromagnetic materials tend to have a non-linear response to magnetic flux, resulting in pickup signals comprising higher-harmonic signals. The core 131 is preferably comprised of a non- ferromagnetic material having a hoUow center. If the core 121 is made of a ferromagnetic material, then it is preferable that the core 131 be made of a similar ferromagnetic material so that the non-linear responses of the cores 121 and 131 of the pickups 120 and 122 substantiaUy cancel. Because the drive coU 130 generates a very uniform magnetic flux along its axis it is preferable that the region of space in which canceUation of magnetic flux is desired be surrounded by the drive coU 130, however, it wiU be appreciated that the region of space in which canceUation is desired may be external to the drive coil 130. It wiU also be appreciated that the second pickup coύ 122 could be wrapped around the core 111 without being interwoven with the drive coU 130 as shown. It will also be appreciated that these methods for canceling magnetic flux may be used along with a device that generates a static magnetic field for canceling external static magnetic fields. Another embodiment for a canceUation circuit of the present invention is shown in FIG. 11. The circuit in FIG. 11 includes a first pickup coU 140 wrapped around a core 141, a second pickup coU 142 wrapped around the core 141, and a drive coU 140 wrapped around the core 141. The first pickup coU 140 is connected to the input of a phase-adjustment circuit 145. The output of the phase-adjustment circuit 145 is connected to the input of an ampUtude-adjustment circuit 144. The second pickup coU 142 is connected to the input of an ampUtude-adjustment circuit 146. It wiU be appreciated that either ampUtude-adjustment circuit 144 or 146 may act only as a buffer, as ampUtude-adjustment of only one of the pickup coU 140 and 142 outputs is necessary. The output of the ampUtude-adjustment circuits 144 and 146 are connected to a combining circuit 148. The output of the combining circuit 148 is connected to a compensation circuit 149. The compensation circuit 149 is connected to an amplifier 156. The amplifier 156 ampUfies its input signal into a drive signal which flows through the drive coU 150, and generates a magnetic flux. The pickup coils 140 and 142 are responsive to the external magnetic flux generated by the drive coil 150. The pickup coils 140 and 142 may be positioned relative to the drive coU 150 as shown in FIG. 11 such that one of the pickup coils, such as pickup coil 140, receives a greater magnitude of the magnetic flux generated by the drive coU 150 than does the second pickup coU 142. Thus when the ampUtudes and phases of the pickup signals from each of the pickup coils 140 and 142 are adjusted so that the contributions of magnetic flux generated by the drive coU 150 cancel when these pickup signals are combined at the combining circuit 148, the combined response of the pickup con's 140 and 142 to uniform external magnetic flux wiU be substantiaUy non-zero. It will be appreciated that other methods may be used to adjust the responses of the pickup coUs 140 and 142 to external magnetic flux such as utilizing different numbers of coil windings in the pickups 140 and 142, or changing the size, shape or material of the core 141 which the pickup coUs 140 and 142 are wrapped around. The compensation circuit 149 may comprise either or both phase-adjustment circuits (not shown) and ampUtude-adjustment circuits (not shown) for adjusting the phase and or ampUtude response of the drive signal so that the drive signal has a specific amplitude and phase relationship to the external magnetic flux impinging on the pickup coUs 140 and 142. The drive signal flows through the drive coil 150 and generates a uniform magnetic flux inside the drive coil 150 that cancels the external magnetic flux inside the drive coil 150. It wiU be appreciated that the circuit shown in FIG. 11 may be used to drive or damp the motion of a ferromagnetic element (not shown) that generates a magnetic flux by moving through a magnetic field. In this case, the core 141 may be made of a ferromagnetic material, and it may be shaped so that both endpoles of the core 141 are in close proximity to the ferromagnetic element (not shown) for providing a more powerful and concentrated driving or damping force to the ferromagnetic element To produce a driving force, the drive coU 150 would preferably generate a magnetic flux that is in-phase with the motion of the ferromagnetic element increasing in strength as the speed of the ferromagnetic element toward the core increases. To produce a damping force, the drive coU 150 would preferably generate a magnetic flux that is out of phase, hence opposing the motion of the ferromagnetic element An embodiment for a canceUation circuit of the present invention is shown in FIG. 12. The circuit in FIG. 12 includes a pickup coU 160 wrapped around a pickup core 161, a signal generator 162, and a drive coU 170 wrapped around a second core 171. The pickup coU 160 is connected to the input of an ampUtude-adjustment circuit 164. The signal generator 162 provides a signal to an amplifier 176 which amplifies the signal for producing a drive signal. The drive signal flows through the drive coil 170 and generates a magnetic flux. The signal generator 162 is connected to the input of a phase- adjustment circuit 167. The output of the phase-adjustment circuit 167 is connected to an ampUtude-adjustment circuit 166. The outputs of the ampUtude-adjustment circuits 164 and 166 are connected to a combining circuit 168. The output of the combining circuit 168 provides a pickup signal that is substantiaUy free from the response of the pickup coU 160 to the magnetic flux generated by the drive coil 170. The canceUation circuit shown in FIG. 12 demonstrates that in order to provide a pickup with a cancellation signal, it is not necessary to have a second pickup device. In fact any electrical representation of a drive signal that has the proper phase and ampUtude characteristics may be used to cancel the response of the pickup to external magnetic flux generated by that drive signal. In this case, the waveform of the drive signal is generated by a signal generator 162. The output of the signal generator 162 is adjusted by a phase-adjustment circuit 167 and an ampUtude-adjustment circuit 166 before it is combined with the output of the pickup coU 160. An ampUtude-adjustment circuit 164 is shown connected to the output of the pickup coU 160 with a resistor R connected to electrical ground. It wUl be appreciated that either ampUtude-adjustment circuit 164 or 166 may act only as a buffer, as ampUtude-adjustment of one of the signals from either the pickup coil 160 or the signal generator 162 is necessary. It witt also be appreciated that the output signal V^ of the combining circuit 168 may be supplied to an input of the signal generator 162 for generating or controlling the frequency and amplitude of the generated signal output to the amplifier 176. The magnetic flux generated by the drive coil 170 induces a voltage V B in the pickup coU 160 which is proportional to magnitude of the magnetic flux. The magnitude of the magnetic flux generated by the drive coil 170 is proportional to the drive current I D in the drive coU 170. The drive current I D = V D /(R D + iωL-,), where R D and L-, are the effective resistance and inductance, respectively, of the drive coU 170, and V D is the drive voltage. The drive voltage V D = G V Q , where G is the gain of the amplifier 176 and V 0 is the signal voltage produced by the signal generator 162. The voltage V p of the pickup coil 160 at the input of the ampUtude-adjustment circuit 164 is V P = V B R / (R + R P + iωL p ) =B V 0 R / (R + R P + iωL p )(R D + -_._--,) where R P and Lp are the effective resistance and inductance, respectively, of the pickup coil 160, and B is a proportionaUty constant that represents the contribution of gains and losses in the circuit The signal voltage at the input of the combining circuit 168 which is connected to the output of the ampUtude-adjustment circuit 164 is Vcp - A V P = A B V 0 R / (R + R p + iωL p )(R D + iωL- j ), where A is the gain of the ampUtude-adjustment circuit 164. In the case where the combining circuit 168 is a differential amplifier circuit it is desirable that the input signal, V co , from the output of the ampUtude-adjustment circuit 166 be substantiaUy identical to V Q . in order for canceUation to occur. One possible design for a circuit that may be used as the phase-adjustment and ampUtude-adjustment circuits 167 and 166, respectively, is two inverting ampUfier circuits connected in series as shown in FIG. 8C The impedance element Z u of the first inverting ampUfier circuit would comprise a resistor (not shown) having the value (R + R p ) and an inductor (not shown) having the value L p , the resistor and inductor being connected in series. The impedance element Z u of the second inverting amplifier circuit would comprise a resistor (not shown) having the value R D and an inductor (not shown) having the value L-y the resistor and inductor being connected in series. The impedance element Z 12 of the first inverting ampUfier 98A, and the impedance element of the second inverting ampUfier 98B would have values such that their product equals the value; A B R. Any buffer amplifiers such as ampUfier 94 would provide unity gain. Thus the input signal V co into the combining circuit 168 wiU be substantiaUy identical to the other input signal V Q ,, and thus wiU cancel. It will be appreciated that the example shown iUustr ates only one of many designs for combined phase and ampUtude-adjustment circuits that may be used as ampUtude and phase-adjustment circuits 166 and 167, respectively. Furthermore, it will also be appreciated that either or both phase and ampUtude-adjustment may be performed on the signal from the pickup coU 160, such as pickup signal V p , in addition to, or instead of the phase-adjustment and ampUtude-adjustment performed on the signal V 0 generated by the signal generator 162. For example, the amplitude-adjustment 164 circuit may be preceded or foUowed by, or comprise a phase-adjustment circuit (not shown) that adjusts the phase of the signal from the pickup coil 160. Another embodiment for a cancellation circuit of the present invention is shown in FIG. 13. The receive and transmit elements in FIG. 13 include a pickup coil 180 wrapped around a pickup core 181, and a drive coil 190 wrapped around a second core 191. The pickup coil 180 is connected to the input of an ampUtude-adjustment circuit 184 at Terminal A. An ampUfier 196 generates a drive signal at Terminal B which flows through the drive coil 190 and generates a magnetic flux. The output of the amplifier 196 is connected to the input of a phase-adjustment circuit 187. Preferably, there is some sort of buffer (not shown) as part of the phase-adjustment circuit 187, such as a large value of resistance that forms a voltage divider with the drive coU 190 for attenuating the input signal to the phase-adjustment circuit 187. The output of the phase-adjustment circuit 187 is connected to an ampUtude-adjustment circuit 186. The outputs of the ampUtude-adjustment circuits 184 and 186 are connected to a combining circuit 188. The output of the combining circuit 188 provides a pickup signal at the input of the ampUfier 196 (Terminal Q that is substantiaUy free from the response of the pickup coU 180 to the magnetic flux generated by the drive coU 190. The canceUation circuit shown in FIG. 13 demonstrates that in order to provide a pickup with a canceUation signal, it is not necessary to have a second pickup device. Rather, part of the drive signal used to generate an external magnetic flux may be combined with a pickup signal to cancel the response of the pickup device to that external magnetic flux. The signal V A at Terminal A represents a pickup signal βV D induced by the magnetic flux generated by the drive coil 190, where V D is the voltage of the drive signal flowing through the drive coU 190 and β is a scaling factor that represents losses between the magnitude of the drive signal V D in the drive coU 190 and the magnitude of the pickup coU's 180 response to that drive signal V D after being are connecte to separate inputs o a combining circuit 208. The combining circuit 208 ux. e signa vo tage at ermina is: B = p , w ere Q is a proportiona ty constant Because the drive coil 200 has a complex impedance Z D = R D + iωL,-, and other circuit elements (not shown) associated with the drive coU 200 may also contribute frequency-dependent terms to the effective impedance of the coil 200, it is necessary that phase-adjustment be performed to compensate for phase-variations between the drive components V D of the signal voltages V A and V B at Terminals A and B, respectively. After phase and ampUtude-adjustment has been performed on either or both of the signals V A and V^ these signals are combined in the combining circuit 208 such that the signal components related to the drive voltage V D cancel, leaving only a signal that is related to the pickup signal V pj ^. This method of canceUation aUows a single unit such as the drive coU 200 shown in FIG. 14A, to simultaneously transmit and receive electromagnetic signals. It wiU be appreciated that the output signal V^ may include an interface (not shown) to the signal generator 202 to control the amplitude, frequency and phase of the signal generated by the signal generator 20Z It witt be further appreciated that if the core 201 is made of a ferromagnetic material, a voltage V H resulting from the non-linear response of that material to the magnetic flux generated by the drive cott 200 wiU be included in the signal voltage V A at Terminal A: V A = V D + + V H . Such nonlinearity creates even harmonics V H in the fundamental driving frequency V D that are not reducible to zero by prior-art canceUation techniques. Thus it may be necessary to eliminate the signal voltage V H resulting from the non-linear response of the core 201 material by providing the circuit with a filter means (not shown) for filtering out these higher harmonic effects, or a harmonic compensation circuit, such as harmonic compensation circuit 210. FIG. 14B shows a design for a circuit that may be used as the harmonic compensation circuit 210. The circuit in FIG. 14B includes a cott 212 wrapped around a core 211 that has a substantiaUy identical non-linear response to magnetic flux as does the core 201. The coil 212 may be oriented with respect to magnetic flux generated by the coil 200 such that the electrical signals induced in the cott 212 by the magnetic flux wiU substantiaUy cancel. The coil 212 may be provided with active magnetic shielding such as a cancellation circuit (not shown), or magnetic shielding materials (not shown) for substantially reducing the response of the cott 212 to magnetic flux generated by external magnetic sources (not shown). The material of the core 211 responds to the magnetic flux generated by the signal output V B of the signal generator 202 at terminal B and substantially reproduces the non-linear effect in the signal V B that will be used to cancel the signal V A . FIG. 14C shows a design for a circuit that may be used as the harmonic compensation circuit 210. The circuit in FIG. 14C includes a spUtting circuit 213 for spUtting the input signal from Terminal B into two signals at outputs Terminals Bl and B2. The output terminal. Terminal B2, is connected to an input terminal. Terminal C2, of a combining circuit 223. Terminal Bl is connected to a harmonic generator circuit 214 which substantially reproduces the shape of the harmonic signal V H in the signal voltage V A at Terminal A. The harmonic generator circuit 214 may include one or more harmonic-generator circuits (not shown) known to persons skilled in the art as "frequency-doublers" or "frequency-triplers." The output of the harmonic-generator circuit 214 is connected to a phase-adjustment circuit 218. The phase-adjustment circuit 218 is connected to an input of an ampUtude-adjustment circuit 219. The output of the ampUtude-adjustment circuit 219 is connected to an input. Terminal Cl, of the combining circuit 223. A signal that is proportional to the drive signal V D is input to the combining circuit 223 at Terminal C2. A signal that is proportional to the harmonic signal V H is adjusted in phase and or ampUtude by the phase-adjustment circuit 218 and the ampUtude-adjustment circuit 219 before being input to the combining circuit 223 at Terminal Cl. The signal voltage V c at the output of the combining circuit 223, Terminal C, has ampUtude and phase relationships between signals V D and V H such that when that signal V c is combined with the ampUtude-adjusted, phase-adjusted signal V A at the combining circuit 218, the contributions of the harmonic terms V H and the drive signal terms V D will substantially cancel. FIG. 14D shows a design for a circuit that may be used as the harmonic compensation circuit 210. The circuit in FIG. 14D includes a spUtting circuit 213 for spUtting the input signal from Terminal B into three signals at outputs Terminals Bl, B2, and B3. Terminal B3 is connected to an input terminal, Terminal C3, of the combining circuit 223. Terminal B2, is connected to the input of an ampUtude- adjustment circuit 216. Terminal Bl is connected to a cott 212 that is wrapped around a core 211. The core 211 is made of a material that has a substantially identical non-linear response to magnetic flux as does the material of the core 201. Thus the output of the coil 212 comprises a signal voltage that has a component that is proportional to the drive signal V D and a component that is proportional to the harmonic signal V H . The cott 212 is connected to the input of a phase-adjustment circuit 221. The output of the phase-adjustment circuit 221 is connected to the input of an ampUtude-adjustment circuit 215. The outputs of the ampUtude-adjustment circuits 215 and 216 are connected to a combining circuit 217, which combines the outputs such that the contributions that are proportional to the drive signal voltage V D substantiaUy cancel, leaving only a signal that is proportional to the harmonic signal V H . The output of the combining circuit 217 is connected to a phase-adjustment circuit 218. The output of the phase- adjustment circuit 218 is connected to the input of an ampUtude-adjustment circuit 219. The output of the ampUtude-adjustment circuit 219 is connected to the input Terminal Cl of the combining circuit 223. The combining circuit 223 combines the harmonic signal V H and the drive signal V D such that their relative proportion and phase are substantially identical to the relative proportion and phase of the harmonic signal V H and drive signal V D in the signal V A . It will be appreciated that the splitting circuit 213 and the combining circuits 217 and 223 shown in FIG. 14C and FIG 14D may control relative amplitudes between the spUt and combined electrical signals, respectively, and thus eliminate the need for ampUtude-adjustment circuits 215, 216, and 219. In FIG. 14C and FIG. 14D the output of the amplitude-adjustment circuit 219 is shown connected to an input terminal of the combining circuit 223, however it wiU be appreciated that the output of the ampUtude- adjustment circuit 219 may be connected to the output V out of the combining circuit 208. The circuit shown in FIG. 15 shows an embodiment of a cancellation circuit of the present invention as it would be used in a simultaneous transmit/ receive system. An electrical signal Vβ at Terminal B is amplified into a drive signal V D at the output (Terminal A) of an amplifier 233. Terminal A is connected to a drive cott 230 that may be wrapped around a core 231. Terminal A is also connected to an ampUtude- adjustment circuit 234, which typicaUy wttl provide attenuation so that the output of the ampUtude-adjustment circuit 234 is substantially lower in ampUtude than the drive signal V D . The output of the ampUtude-adjustment circuit 234 is connected to the input of a phase-adjustment circuit 235. Terminal B is connected to the input of an ampUtude- adjustment circuit 236. The output of the ampUtude-adjustment circuit 236 and the output of the phase-adjustment circuit 235 are connected to a combining circuit 238. The output of the combining circuit 238 is connected to a preamplifier 232. The output of the preamplifier 232 is connected to Terminal B. The signal-voltage V A at Terminal A comprises a drive voltage V p , which flows through the drive cott 230 to generate a magnetic flux, and a pickup voltage V p ^ that is induced in the drive coil 230 by other sources (not shown) of magnetic flux. In this case, it is preferable that the material comprising the core 231 has a substantiaUy linear response to the magnetic flux so as to minimize the additive harmonic signature V H that is caused by non-linear responses of the core 231 material to magnetic flux and would otherwise be included in the signal voltage V A . The signal-voltage V B at Terminal B represents the drive signal V D before it is amplified by the ampUfier 233. The phase- adjustment circuit 235 and the ampUtude-adjustment circuits 234 and 236 adjust the relative phase and amplitude of the signals V A and V B so that the components in the signals V A and V B that are related to the drive signal V D can be made to substantiaUy cancel when they are combined at the combining circuit 238. The total gain of a feedback loop is calculated by assessing the gains and losses of each component in the feedback loop. For example, the gain of the first feedback loop in FIG. 15 is assessed starting at Terminal B, moving through the ampUtude-adjustment circuit 236 to the combining circuit 238, then through the preamplifier 232 back to Terminal B. The ampUtude-adjustment circuit 236, and the preamplifier 232 may provide gain or attenuation to the electrical signals. The combining circuit 238 provides an effective attenuation to the electrical signals by virtue of it canceling the electrical signals representing the drive signal V D . Thus if the total gain of this first feedback loop is less than one, then this part of the circuit wiU not cause oscillation. The gain of the second feedback loop in the circuit shown in FIG. 15 is assessed starting at Terminal A, going through the amplitude-adjustment circuit 234 and the phase-adjustment circuit 235 through the combining circuit 238 to the preampUfier 232, and finaUy, through the ampUfier 233 returning to Terminal A. The ampUtude- adjustment circuit 234 includes a means for attenuating the signal V A at Terminal A. The preamplifier 232 and the amplifier 233 will typicaUy provide substantial gain to the electrical signal, while the phase-adjustment circuit 235 will usuaUy have Uttle effect on the amplitude of the electrical signal. The combining circuit 238 will provide a substantial effective attenuation to the electrical signal flowing through it by virtue of it canceling the signal-voltage that is related to the drive signal V D . If this canceUation is large enough, it will cause the total gain of the second feedback loop to be less than one, therefore the circuit will not osciUate. is amplified and feedback effects of the drive signal V D in the circuit are canceled to prevent oscttlation. This aUows the drive coil 230 to transmit and receive an electromagnetic signal simultaneously. It wttl be appreciated that if the core 231 is made of a material that has a non-linear response to magnetic flux, the cancellation circuit may utilize a design to cancel the electrical signals resulting from the non-linear response of the core 231 material, such as is shown in FIG. 14B through FIG. 14D. It wttl also be appreciated that either or both of the signals V A and V B from Terminals A and B, respectively, may have ampUtude and or phase-adjustment applied to them such that the signal components related to the drive signal V D wttl cancel at the combining circuit 238. Furthermore, it wttl be appreciated that a compensation circuit (not shown) may be included in the feedback loop or preceding the drive cott 230 in order to provide a specific phase and or ampUtude relationship between the pickup signal V p ^ p and the drive signal V D . The circuit shown in FIG. 16 is an embodiment of a canceUation circuit of the present invention that cancels both static magnetic fields and magnetic flux in a specific region of space. The circuit in FIG. 16 includes two magnetic field sensors 240 and 242, each of which generates an electrical signal that is proportional to the scalar magnitude of the magnetic field strength in a specific direction at the location of each sensor 240 and 242. The sensors 240 and 242 may be flux gate sensors, or the Uke. The signal from the first sensor 240 is adjusted in phase by a phase-adjustment circuit 241, and ampUtude by an ampUtude-adjustment circuit 244. The signal from the second sensor 242 is adjusted in phase by a phase-adjustment circuit 243, and ampUtude by an ampUtude-adjustment circuit 246. The outputs of the ampUtude-adjustment circuits 244 and 246 are connected to a combining circuit 247. The output of the combining circuit 247 is connected to a compensation circuit 248, which is connected to the input of an amplifier 249. The ampUfier 249 amplifies an input signal into a drive signal V D that flows through the drive coil 250 and produces a magnetic field that is substantiaUy paraUel, though opposite to the magnetic field sensed by the sensors 240 and 24Z The first sensor 240 is positioned in close proximity to the cott 250, or inside the cott 250 in order to sense both the magnetic field generated by the cott 250 and the magnetic field generated by external sources (not shown). The second sensor 242 is positioned in such a manner so that it is sensitive to the magnetic fields generated by external sources (not shown), but not as sensitive to the magnetic field generated by the cott 250 as is the first sensor 240. Each of the sensors 240 and 242 has a specific ratio of response between the magnetic field generated by the coil 250 and the magnetic field generated by the external magnetic sources (not shown). It wttl be appreciated that there are many ways to change the ratio of response of one of the sensors 240 or 242, adjusting the position being only one of the ways, however the important point is to provide one of the sensors 240 or 242 with a different ratio of response than the other sensor 242 or 240. The amplitude and or phase of the signals produced by the sensors 240 and 242 are adjusted by ampUtude-adjustment circuits 244 and 246, respectively, and phase-adjustment circuits 241 and 243, respectively, such that the components of those signals that are related to the drive signal V D will substantiaUy cancel when they are combined at the combining circuit 247. However, the output of the combining circuit wttl comprise a voltage VE*. that is proportional to the magnetic field intensity generated by the external sources (not shown). The ampUtude of the signals produced by the sensors 240 and 242 will typically need to be adjusted in order to cancel the dc signal resulting from the magnetic field generated by the drive coil 250. However, as the magnitude of the drive signal V D changes in response to a changing external magnetic field, there may be some response anomaUes between the two sensors 240 and 242 related to the rate-of-change (flux) of the drive signal V D . Thus it may be necessary to compensate for flux-dependent ampUtude and rate-of-response (phase) differences between the two sensors 240 and 242. The drive cott 250 generates a highly uniform magnetic field within the region of space that it encloses. Thus it is preferable to utilize the interior of the cott 250 as the space in which uniform magnetic fields wttl be canceled. However, due to the inductive properties of the cott 250 and the possible flux-dependent amplitude and phase characteristics of the sensors 240 and 242, it is necessary to provide phase and or amptitude-compensation using a compensation circuit, such as compensation circuit 248, in order to provide substantial cancellation of magnetic flux. The circuit shown in FIG. 17 is another embodiment of a canceUation circuit of the present invention that cancels both static magnetic fields and magnetic flux in a specific region of space. The circuit in FIG. 17 includes a magnetic field sensor 262 which generates an electrical signal that is proportional to the scalar magnitude of magnetic field strength in a specific direction at the location of the sensor 262. It wttl be appreciated that the sensr 262 may be a flux gate sensor, or the like. The signal produced by the sensor 262 is sent to an automatic control unit 264. The automatic control unit 264 controls the gain of an amplifier 266 connected to a dc-level generator 265. The output of the ampUfier 266 is an ampUfied or attenuated dc-level drive signal V D that is passed through a compensation circuit 267 to a drive cott 260, which generates a magnetic field. The magnetic field sensor 262 preferably is positioned inside the region of space where canceUation of magnetic fields is desired. The magnetic field sensor 262 produces a signal that is proportional to the ampUtude of the magnetic field it senses, which comprises the magnetic field generated by the drive cott 260 and magnetic fields generated by other sources (not shown). The automatic control unit 264 determines if a magnetic field is present at the sensor 262 and controls the amplifier 266 so that the drive signal V D in the drive cott 260 produces a magnetic field that cancels the magnetic field at the sensor 262. Because the drive cott 260 has inductive properties, there wttl be an inductive lag in the drive signal V D flowing through the drive cott 260 when the ampUtude of that signal changes. Likewise, the effective impedance Z D = R D + iωl-- . of the cott 260 changes with signal frequency ω, thus a signal flux wttl result in a variation of the ampUtude of the magnetic field generated by the drive cott 260. The compensation circuit 267 provides phase and ampUtude-adjustment to the drive signal V D so that the drive cott 260 is able to provide substantial canceUation of both static and dynamic magnetic fields. It wttl be appreciated that the compensation circuit 267 may also provide compensation for any flux-dependent ampUtude and phase variations in the response of the sensor 262. It wiU also be appreciated that the automatic control unit may include a means for providing ampUtude and phase-compensation to the drive signal V D . The circuits shown in FIG. 16 and FIG. 17 show methods of canceling magnetic fields along a single axis, however, a superposition of three such circuits, each along an orthogonal axis will provide complete cancellation of magnetic fields in three dimensions. Because coils of wire whose currents support magnetic fields in space function as antennas radiating electromagnetic energy, it is obvious that the canceUation and or compensation circuits shown above may be used in radar systems for providing noise- cancellation and simultaneous transmit/ receive capability. The circuit shown in FIG. 18 is a preferred embodiment of a cancellation circuit of the present invention. A signal generator 276 generates an electrical generator signal V G that is ampUfied by a power ampUfier 274 and passed through a junction 272 to an antenna element 270, which both emits and receives electromagnetic radiation. The antenna element 270 is responsive to other sources (not shown) of electromagnetic radiation, and produces an electrical pickup signal V P . The junction 272 is connected to an input of a combining circuit 275, which receives the pickup signal V p along with a leakage signal V from the power amplifier 274, which is a portion of the generator signal V G . The signal generator 276 also produces a reference signal V R that is similar in shape to the generator signal V G . The reference signal V R passes through an ampUtude- adjustment circuit 271 and a phase-adjustment circuit 273 to an input of the combining circuit 275. The output of the combining circuit 275 is amplified by an ampUfier 278. The junction 272, which the generator signal V G passes through on its way to the antenna element 270 may be a circulator (not shown) which directs most of the power from the power amplifier 274 to the antenna element 270. However, because the efficiency of a circulator is frequency-dependent on the electrical signals passing through it the performance of the circulator (not shown) wttl be degraded by the use of large signal-bandwidths or multiple frequencies as weU as other operational limitations. Therefore some of the energy from the power amplifier 274 wttl leak into the combining circuit 275. The ampUtude-adjustment circuit 271 wttl provide frequency-dependent ampUtude-adjustment to the reference signal V R such that its ampUtude is substantiaUy identical to the ampUtude of the leakage signal V L leaked from the power amplifier 274 into the combining circuit 275. It will be appreciated that the ampUtude-adjustment circuit 271 may be a circulator that is similar to the circulator used as the junction 272. The output of the ampUtude-adjustment circuit 271 would then be substantiaUy proportional to the ampUtude of the leakage signal V L . The phase- adjustment circuit 273 adjusts the phase of the reference signal V R such that it wttl cancel the leakage signal V L at the combining circuit 275. Preferably, the phase- adjustment circuit 273 wttl produce a substantiaUy constant phase between the leakage signal V and the reference signal V R over the desired frequency range of generated signals V G . Thus the output of the combining circuit 275 wttl comprise a pickup signal V P that is substantiaUy free from the effects of leakage signal V emanating from the power amplifier 274. It wttl be appreciated that many possible designs exist for canceUation circuits to cancel the effects of signal interference between the transmitting and receiving elements of a radiating system, the circuit shown in FIG. 19 being only one of these designs. AmpUtude and or phase-adjustment circuits (not shown) may be interposed in the circuit between the junction 272 and the combining circuit 275. Furthermore, in the case where ampUtude-modulated and or frequency-modulated signals are generated and received, the circuit may include filters (not shown) for filtering out the carrier frequency before the cancellation circuit removes the transmitted signal from the received signal. The phase-adjustment circuit 273 may include a delay apparatus (not shown), such as delay lines, for delaying part of the reference signal V R such that some of the transmitted radiation emanating from the antenna element 270 that reflects back from nearby objects, such as ground clutter, may be canceled from the pickup signal V p . The reference signal V R may also include electrical signals that are similar in shape to signals induced by other noise sources (not shown) in the antenna element 270, and separate ampUtude and phase-adjustment may be appUed for canceling the response of the antenna element 270 to these other noise sources (not shown). Furthermore, the antenna element 270 may be responsive to incident radiation for producing a drive signal that aUows an antenna, such as the antenna element 270, to transmit electromagnetic radiation that cancels the reflection of the incident radiation off of the antenna element 270 or some other object (not shown). Thus the reflected radiation may be canceled at a distant receiver (not shown). Magnetic pickups comprising pickup coils are shown in the circuits of FIG.2 through FIG. 15, however any type of magnetic pickup may be used with these types of canceUation circuits. Likewise, a compensation circuit may be used to compensate for ampUtude and phase anomaUes arising from any pickup device that produces an electrical pickup signal, and a canceUation circuit may be used to cancel the electrical signals arising from the response of the pickup device to noise. For example, a canceUation circuit may be used for canceling the signals generated by an optical sensor's electrical response to background electromagnetic radiation. In many of the figures, a drive cott is illustrated as the element that generates an electromagnetic field, however, compensation circuits may be used to compensate for frequency-response characteristics exhibited by any structures which generate electromagnetic fields. The preferred methods of ampUtude-adjustment were shown to be electrical gain and attenuation controls. However, it wttl be appreciated that other methods of ampUtude-adjustment may be used, such as position-adjustment of the pickup coils, the drive coils, the cores for the pickup coils and drive coils, or nearby permeable and or conducting materials. It wttl also be appreciated that the inductance of a coil may be changed by changing the reluctance of the path seen by that coil's magnetic field. The magnitude of electrical current in the pickup coils was considered to be very smaU, thus the formulation of the equations representing the electrical pickup signals induced in the pickup coils by magnetic flux have not included the inductive effects that the pickup coils may have on each other. However, the scope and spirit of the present invention would not be chaUenged by considering the inductive effects between pickup coils when designing the canceUation circuits. Furthermore, consideration of the more subtle electromagnetic effects, such as how capacitance in the pickup coils affects the induction of electrical signals in the pickup coils and the canceUation and compensation circuits that may be designed accordingly, has been clearly anticipated by this invention. Although the invention has been described in detail with reference to the ttlustrated preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and as defined in the foUowing claims.

Claims: 1. A canceUation circuit for canceling inductive noise in electrical circuits resulting from magnetic flux, the canceUation circuit comprising a first magnetic pickup means responsive to external magnetic flux for producing a first pickup signal, a second magnetic pickup means responsive to external magnetic flux for producing a second pickup signal, an ampUtude-adjustment means for adjusting the amplitude of either or both said first and second pickup signals for substantiaUy equalizing the ampUtudes of inductive noise in said first and second pickup signals, a phase-adjustment means for adjusting the relative phase between said first and second pickup signals to a predetermined phase-relationship such that when said first and second pickup signals are combined, said inductive noise substantiaUy cancels, and a combining means to combine said first and second pickup signals for producing a combined pickup signal that is substantiaUy free from inductive noise. 2. The canceUation circuit of claim 1 wherein said predetermined phase-relationship is 0 degrees or 180 degrees. 3. The canceUation circuit of claim 1 wherein said phase-adjustment means and said ampUtude-adjustment provide substantial canceUation of inductive noise over a broad range of signal-frequency. 4. The canceUation circuit of claim 1 wherein said phase-adjustment means and said ampUtude-adjustment provide substantial canceUation of inductive noise over a narrow range of signal-frequency. 5. The canceUation circuit of claim 1 wherein said phase-adjustment provides a predetermined phase-shift to said first and second pickup signals for providing said combined pickup signal with a predetermined phase-shift 6. The canceUation circuit of claim 1 further comprising an ampUfier means for amplifying said combined pickup signal into a drive signal, and a magnetic drive means for receiving said drive signal and generating a magnetic flux. 7. The canceUation circuit of claim 1 further comprising a compensation means for providing a specific frequency-dependent ampUtude and or phase to said combined pickup signal. 8. The cancellation circuit of claim 6 further comprising a compensation means for providing a specific frequency-dependent ampUtude and or phase to said combined pickup signal. 9. The cancellation circuit of claim 1 wherein said first magnetic pickup means comprises a first pickup coil wrapped around a core, and said second pickup means comprises a second pickup coil wrapped around said core. 10. The canceUation circuit of claim 6 wherein said magnetic drive means comprises a first drive cott wrapped around a core, and a second drive cott wrapped around said core. 11. The canceUation circuit of claim 10 wherein said core is curved or otherwise shaped such that the magnetic field generated by said magnetic drive means is substantiaUy concentrated and confined to a predetermined region of space. 12. The canceUation circuit of claim 6 wherein said first magnetic pickup means comprises a first pickup cott wrapped around a core, and said magnetic drive means comprises a drive cott wrapped around said core. 13. The canceUation circuit of claim 12 wherein said second magnetic pickup means comprises a second pickup cott wrapped around said core. 14. A canceUation circuit for a simultaneous transmit/receive system for canceling electromagnetic noise in a receiver caused by electromagnetic signals generated by a transmitter, the canceUation circuit comprising a signal generator means for producing a generated electrical signal, an amplifier means for amplifying said generated electrical signal into an amplified electrical signal, a transmitter means for receiving said amplified electrical signal and producing an electromagnetic transmit signal, a receiver means responsive to electromagnetic radiation for producing a receive signal that incudes a receiver response to said electromagnetic transmit signal, a referencing means for providing a reference signal to be combined with said receive signal, said reference signal being similar to said electromagnetic transmit signal, an ampUtude-adjustment means for adjusting the ampUtude of either or both said reference signal and said receive signal such that said reference and receive signal ampUtudes are substantially equal, a phase-adjustment means for adjusting the relative phase between said reference and receive signals, and a combining means for combining said ampUtude-adjusted, phase-adjusted reference and receive signals into a combined signal thereby substantiaUy canceling the contributions of the electromagnetic transmit signal to the receive signal. 15. The canceUation circuit of claim 14 wherein said transmitter means and said receiver means share the same structure for generating and receiving electromagnetic radiation. 16. The cancellation circuit of claim 14 wherein said referencing means comprises a spUtting circuit for spUtting part of said generated electrical signal or said ampUfied electrical signal or said electromagnetic transmit signal to produce said reference signal. 17. The canceUation circuit of claim 14 wherein said referencing means comprises a reference signal generator means for generating said reference signal. 18. The cancellation circuit of claim 14 wherein said referencing means comprises a second receiver means responsive to said electromagnetic transmit signal. 19. The canceUation circuit of claim 17 wherein said reference signal generator means is coupled to said signal generator means for controlling relative phase and or ampUtude between said reference signal and said generated electrical signal. 20. The canceUation circuit of claim 14 wherein said reference signal further includes external reference signals, said external reference signals being similar to the response of said receiver to external sources or reflectors of electromagnetic radiation. 21. The canceUation circuit of claim 14 wherein said signal generator means comprises a connecting means, the input of said connecting means being connected to the output of said combining means, the output of said connecting means providing said generated electrical signal to said ampUfier means. 22. The cancellation circuit of claim 14 further comprising a harmonic-compensation means for canceling the non-linear response of ferromagnetic materials to magnetic flux. 23. The canceUation circuit of claim 14 further comprising a compensation means for compensating for frequency-dependent phase and or ampUtude variations between said receive signal and said electromagnetic transmit signal. 24. The canceUation circuit of claim 23 wherein said compensation means provides a predetermined phase-relationship between said receive signal and said electromagnetic transmit signal. 25. The canceUation circuit of claim 24 wherein said predetermined phase-relationship is 0 degrees or 180 degrees. 26. A canceUation circuit for canceling magnetic fields within a predetermined region of space, the canceUation circuit comprising a magnetic sensor means responsive to external magnetic fields for generating an electrical pickup signal that is substantiaUy proportional in magnitude to said external magnetic fields, a drive means for receiving said electrical pickup signal and generating a canceUation magnetic field in said predetermined region of space that substantiaUy cancels said external magnetic fields, and a compensation means for compensating for rate-of-change-dependent ampUtude and or phase variations between said electrical pickup signal and said canceUation magnetic field.

Inventors: SHATTIL STEVEN JAMES, SHATTIL, STEVEN, JAMES
Applicants: SHATTIL STEVEN JAMES, SHATTIL, STEVEN, JAMES
Application year: 1994
Main classification: H05K9
IPC codes: G01R33, H05K9